A commonly traded liquefied hydrocarbon stream contains, or essentially consists of, liquefied natural gas (LNG).
Natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored at high pressures.
Especially for long distance transportation, the liquefied natural gas can be carried in a sea-going vessel between, for example, an export terminal and an import terminal. At an import terminal, the LNG is regasified, and the cold energy can be used to help liquefy nitrogen gas. On its return journey, the sea-going vessel can transport the liquid nitrogen, whose cold energy can then be used in the liquefaction of natural gas.
GB 2 172 388 A describes using liquefied natural gas that has been liquefied off-shore at the wellhead, to liquefy nitrogen in a land-based import plant. The same vessel is used to transport liquefied nitrogen and liquefied natural gas in opposite directions between the land-based plant and the off-shore wellhead.
However, a problem with GB 2 172 388 A is that a small recycling refrigerating liquefaction plant is necessary at the wellhead to top-up the cooling effect of the nitrogen. It appears quite inconvenient to operate and/or maintain such a recycling refrigerating liquefaction plant at such an inconvenient location as an offshore wellhead.